Has anyone measured any of the balls at competition to see what the actual variance is? They didn't seem to vary too much to me, but then again I can't really eyeball differences in circumference that well.
For design purposes though, I averaged out the Soccer balls to have a 8.75" diameter, which is about a 27.5" Circumference. This worked well for me, as competition soccer balls should vary less than 3/16" +/- from this 8.75" Dimension.